Electro-dynamic conical loudspeaker



April 14, 1964 w, HQFMAN 3,129,298

ELECTRO-DYNAMIC CONICAL LOUDSPEAKER Filed May 8. 1961 3 MAGNETS MAGNETS FIG. 1

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lNvENTO R WYTZE1HOFMAN.

AGENT;

United States Patent 3,129,298 ELECTRG-DYNAMKC CONICAL LOUDSPEAKER Wytze Hofman, Eindhoven, Netherlands, assignor to North American Philips Company, Inc, New York,

N.Y., a corporation of Delaware Filed May 8, 1961, Ser. No. 108,346 Claims priority, application Netherlands May 10, 1960 4 Claims. (Cl. 179-120) The invention relates to an electro-dynamic conical loud speaker and has for its object to provide a flatshaped structure thereof, so that the loudspeader can be accommodated in a shallow cabinet. In apparatus equipped with transistors instead of electron tubes the required depth of the cabinet is determined only or mainly by the size of the loudspeaker in the direction of depth, so that a maximum flatness of the loudspeaker is very important.

In accordance with the invention the loudspeaker comprises an annular magnet and cylindrical core supported on a soft iron foot plate and a soft iron lid plate supported on the annular magnet. The lid plate has a centrol concavity projecting towards the foot plate and constituting, together with the said central core, the air gap. The speaker diaphram or cone is thus completely accommodated in the space circumscribed by the annular magnet and associated foot and lid plate.

Excess magnetic leakage is found where the lid plate and the foot plate approach each other; which may be avoided by placing a flat, annular permanent magnet between the adjacent parts of the lid and foot plates polarized to support the field of the annular magnet in the air gap.

It also the core is formed by a permanent magnet, of which the magnetisation direction is opposite that of the adjacent annular magnet, the height of the latter magnet can be reduced, so that an even greater flatness of the loudspeaker is obtained.

Finally, in accordance with the invention, both the lid plate and the foot plate may be provided with one or more perforations in order to reduced the acoustical rigidity of the small space behind the speaker cone to a suitable value.

The invention will now be described more fully with reference to the drawing, in which FIGS. 1 and 2 illustrate cross sectional views of two different embodiments of the invention.

The loudspeaker shown in FIG. 1 comprises a soft, iron disc or foot plate 1, on which, a central cylindrical iron core 2 and peripherally an annular, permanent magnet 3 are arranged. This magnet may comprise an unbroken, or as an alternative, it may consist of two or more parts comprising a ring. For the sake of simplicity the magnet, whether of one or more pieces will be denoted hereinafter as a ring.

Located on the ring 3 is a soft iron lid plate 4, having a central concavity which approaches the foot plate. A central circular opening in the plate 4 surrounds the top end of the core 2, so that an air gap is formed, in which the coil 5 of the cone 6 is movable.

Arranged in the space between the flat central part of the lid plate 4 and the foot plate 1 is an annular permanent magnet 7, which is magnetized in the same direction as the ring 3, which is indicated in the drawing by arrows at the side of the magnets. The ring 7 is provided to avoid excessive magnetic leakage between the central part of the lid plate and the foot plate lying underneath it and also to contribute to formation of a dust-tight closure of the air gap and of the coil 5 movable therein. Complete formation of this closure is obtained by means of a resilient diaphragm 8, by which the cone is partly supported on the lid plate, and by means of a hood 9 of paper or the like forming part of the cone.

Since the air gap 10 behind the cone 6 is comparatively small, the acoustical rigidity of this space has a large value, which may be troublesome under certain conditions owing to an excessive damping of the low tones. In order to mitigate this, one or more perforations 11 may be provided in the lid plate and, one or more perforations 12 may, in addition, he provided in the foot plate. The latter perforations are, of course, only useful, when the magnet 3 is formed by a full ring, so that the space between this ring and the ring 7 is not in open communication with the ambient atmosphere.

The loudspeaker shown in FIG. 2 differs from that shown in FIG. 1 only in as much as the central core, made of soft iron in FIG. 1, is replaced by a permanent magnet 13, which is magnetized in a direction opposite that of the ring 3. Part of the required magneto motive force is thus supplied by the core 13, so that a smaller height of the annular magnet 3 will be attainable. The favorable effect with the device shown in FIG. 2 therefore is that with the same power, the loudspeaker may exhibit an even shallower profile or flatness than that of HG. 1.

What is claimed is:

1'. A loudspeaker comprising an axially polarized permanent magnet of annular configuration, a soft iron disc shaped foot plate supporting said magnet, a magnetizable core on said foot plate concentric with said magnet, a conical soft iron lid plate supported on said magnet, said lid plate having a central concavity extending toward said foot plate and spaced therefrom, a central aperture in said lid plate defining an air gap with said core a voice coil Within said air gap, a speaker diaphragm sup porting said voice coil, said diaphragm secured to said lid plate and receivable within said central concavity, a second magnet in the space between said foot plate and the adjacent portion of said lid plate having the same polarity as said annular magnet, and means for venting the space between said diaphragm and said lid plate.

2. A loudspeaker according to claim 1 with the addition of a membrane extending from the apex of said diaphragm to the adjacent portion of said lid plate, said membrane defining a shield against entry of dirt into said air gap.

3. A loudspeaker according to claim 1 wherein said core member is a permanent magnet having a polarity opposite the polarity of said annular magnet.

4. A loudspeaker comprising an axially polarized permanent magnet of annular configuration, a soft iron disc shaped foot plate supporting said magnet, a magnetizable core on said foot plate concentric with said magnet, a conical soft iron lid plate supported on said magnet, said lid plate having a central concavity extending toward said foot plate and spaced therefrom, a central aperture in said lid plate defining an air gap with said core a 3 voice coil Within said air gap, a speaker diaphragm supporting said voice coil, said diaphragm secured to said lid plate and receivable within said central concavity, a second magnet in the space between said foot plate and the adjacent portion of said lid plate having the same polarity as said annular magnet, and apertures in said lid plate for venting the space between said diaphragm and said lid plate and a disc shaped membrane secured to the apex of said speaker diaphragm and the adjacent apex portion of said lid plate for providing a shield against entry of dirt into said gap.

References ited in the file of this patent UNITED STATES PATENTS 2,085,721 Warnke June 29, 1937 2,256,689 Quam Sept. 23, 1941 2,498,825 Richards Feb. 28, 1950 2,862,069 Marchand et al. Nov. 25, 1958 

1. A LOUDSPEAKER COMPRISING AN AXIALLY POLARIZED PERMANENT MAGNET OF ANNULAR CONFIGURATION, A SOFT IRON DISC SHAPED FOOT PLATE SUPPORTING SAID MAGNET, A MAGNETIZABLE CORE ON SAID FOOT PLATE CONCENTRIC WITH SAID MAGNET, A CONICAL SOFT IRON LID PLATE SUPPORTED ON SAID MAGNET, SAID LID PLATE HAVING A CENTRAL CONCAVITY EXTENDING TOWARD SAID FOOT PLATE AND SPACED THEREFROM, A CENTRAL APERTURE IN SAID LID PLATE DEFINING AN AIR GAP WITH SAID CORE A VOICE COIL WITHIN SAID AIR GAP, A SPEAKER DIAPHRAGM SUPPORTING SAID VOICE COIL, SAID DIAPHRAGM SECURED TO SAID LID PLATE AND RECEIVABLE WITHIN SAID CENTRAL CONCAVITY, A 